Abstract
Background
Globally, noroviruses cause infections year-round but have recognized winter seasonality in the northern hemisphere and yearly variations in incidence. With candidate norovirus vaccines in development, understanding temporal and geographic trends in norovirus disease is important to inform potential vaccination strategies and evaluate vaccine impact.
Methods
We analyzed data from the National Outbreak Reporting System (NORS) and CaliciNet on single-state norovirus outbreaks that occurred from August 2009–July 2019 in the contiguous United States. We defined norovirus season onset and offset as the weeks by which 10% and 90% of norovirus outbreaks in a surveillance year occurred, respectively, and duration as the difference in weeks between onset and offset. We compared norovirus seasons across surveillance years and geographic regions.
Results
During August 2009–July 2019, 24,995 single-state norovirus outbreaks were reported to NORS and/or CaliciNet. Nationally, median norovirus season duration was 24 weeks, with onset occurring between October–December and offset occurring between April–May. Across all years combined, we observed a west-to-east trend in seasonality, with the earliest onset (October) and latest offset (May) occurring in western regions and the latest onset (December) and earliest offset (April) occurring in northeastern regions.
Conclusions
Timing and duration of the US norovi rus season varied annually, but generally occurred during October–May. Norovirus wintertime seasonality was less distinct in western regions and was progressively more pronounced moving east. Further understanding the drivers of spatiotemporal dynamics of norovirus could provide insights into factors promoting virus transmission and help guide future interventions.
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